The present invention relates to novel polymerizable compounds which are useful for liquid crystal cells, retardation films and the like. And the present invention also relates to optical compensators comprising polymerized the compounds.
Liquid crystal compounds should meet some conditions such as having proper melting points and wide temperature ranges of liquid crystal phase for usage of liquid crystal displaying cells, optical compensators and so forth. Various compounds have been synthesized depending on such usage.
Some optical films produced by polymerization of liquid crystal compounds having a polymerizable group are disclosed in JP-A No.07-110469 (the term xe2x80x9cJP-axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent application), JP-A No. 11-80081, xe2x80x9cJ. Appl. Phys. Vol. 79(1996), No. 10, 80988105xe2x80x9d and so forth. According to them, the optical films can be produced by a method comprising a step of melting a compound having a polymerizabble group on a substrate, a step of orienting the compound, and a step of polymerizing the oriented compound. However, since the compounds disclosed in them have so high melting points, it is often the case that the substrates (such as polymer film) having the compounds thereon would distort because of heat. Thus, it is difficult to produce the films through the orienting and polymerizing process.
The compounds, having a small wavelength dependence of refractive indices, disclosed in them also have strong tendencies to form smectic phases, in which the compounds are hard to be optically uniformly oriented, rather than to form a nematic phase, in which the compounds are easy to be optically uniformly oriented, due to a narrow temperature range of the nematic phase. Thus, it is difficult to produce the films having enough optical uniformity as optical compensator films. Providing polmerizable liquid compounds, which have a low melting point and a wide temperature range of a nematic phase, may benefit the technical field of producing optical compensators, like as the material of substrates are free from limitations.
The present invention, devised in light of the various above-described problems, has for its object to provide a polymerizable compound having a low melting point and a wide temperature range of nematic liquid phase. Another object of the present invention is to provide an optical compensator having uniform optical property such as uniform retardation.
In one aspect, the invention provides a compound represented by Formula (I): 
where A1 to A4 are respectively hydrogen or substituents and one of A1 to A4 is halogen, alkyl group, alkoxy group, formyl or cyano a substituent; B1 and B2 are respectively substituents and at least one of them is a substituent containing acryloyloxy, methacryloyloxy or epoxy.
As preferred embodiment, there are provided the compound wherein both of B1 and B2 contain acrylate or methacrylate; the compound wherein at least one of A1 to A4 is fluorine, chlorine, or methyl; the compound wherein at least one of A1 to A4 is chlorine; the compound wherein both of A1 and A3 are hydrogen and both of A2 and A4 are substituents; the compound wherein the compound exhibits liquid crystallinity.
In one aspect, the present invention provides an optical compensator comprising a layer formed from a compound represented by Formula (I): 
where A1 to A4 are respectively hydrogen or substituents and at least one of A1 to A4is halogen, alkyl group, alkoxy group, formyl or cyano; B1 and B2 are respectively substituents and at least one of them is a substituent containing acryloyloxy, methacryloyloxy or epoxy.